Reduction of pressure induced temperature influence on the speed of sound in a gas

ABSTRACT

An apparatus for determining the proportion of gases in a gas mixture, has a measurement chamber having a chamber defining structure, a gas inlet and a gas outlet, an ultrasound source and an ultrasound detector mounted such that the ultrasound source is capable of transmitting ultrasound through the chamber to the ultrasound detector; a temperature sensor mounted such that the sensor is capable of sensing the temperature in the chamber. The chamber defining structure is adapted to amplify thermal exchange with a gas content in the chamber so as to suppress a temperature change in the chamber.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates in general to the measurement of speed ofsound in a gas mixture. More particularly, the present invention relatesto applications of ultrasound detectors for example in the measurementof gas concentration or gas flow in environments where pressure inducedtemperature influence is a dominant factor for measurement accuracy.

In some applications of ultrasound detection, such as the measurement ofconcentration or proportions of gas components in a gas mixture,pressure variations in the gas mixture has a large influence on themeasurement accuracy. There is a well known connection between the speedof sound and gas specific parameters, and, based on this connection andmeasurement values of the speed of sound, a current proportion of gascontent can be calculated. Furthermore, the speed of sound in a gas hasa strong dependency on temperature, and in order to correctly calculatethe gas content a timely and accurate temperature measurement on the gasis also required.

In medical breathing apparatuses it is of vital importance that gasproportions in for example inspiration or expiration air from a patientis accurately determined for the purpose of monitoring and controllingthe dosing of gas components in a gas mixture or for monitoring thehealth state of a patient. However, when a patient is breathing,significant changes in pressure occur in the breathing apparatus andconsequently, in accordance with the ideal gas law, the temperature ofthe gas varies largely. Thus, in order to accurately determine theproportions of the gas content it is generally required that a veryprecise temperature measurement is carried out, and that the temperaturemeasurement and the sound speed measurement are carried out closely intime such that they describe the momentary physical state of the gas.

When measuring temperature in actual practice there is always a certaindegree of delay in the temperature measurement in relation to the realcurrent temperature. The delay depends on the time constant of thetemperature sensor that is used for the measurement. The delay in theultrasound sensor depends on the sampling frequency and is in general soshort that it is insignificant and negligible in comparison with thetemperature measurement. A concurrent measurement of sound speed andtemperature will therefore always result in a, to some degree, erroneoustemperature that in its turn causes an erroneously calculated gasconcentration.

A number of different approaches are known to deal with this measurementproblem. Examples are described in the following publications, which areall incorporated by reference in the present application.

In the technical report A SONAR BASED TECHNIQUE FOR THE RATIOMETRICDETERMINATION OF BINARY GAS MIXTURES, G. Hallewell et al, NuclearInstruments and Methods in Physics Research A264 (1988) 219-234,North-Holland, Amsterdam, there is a theoretical background to this kindof measurement.

U.S. Pat. No. 6,202,468 discloses an apparatus and a method fordetermining the relative proportions of gases in a mixture by measuringmagnetic susceptibility and speed of sound.

U.S. Pat. No. 4,155,246 discloses a gas analyzing system using sonicwave shift over a tubular gas column. U.S. Pat. No. 4,932,255 disclosesa method and device for measuring on a gas flow using surface acousticwaves over a substrate positioned in the gas flow. Thermally conductivepaths around the substrate periphery reduce thermal gradients. In thispiece of prior art the thermally conductive paths are devised in orderto reduce thermal gradients that are created by the sensor in connectionwith transmission of surface acoustic waves.

U.S. Pat. No. 5,351,522 discloses an ultrasound detection based gassensor with an L-shaped measurement chamber. This piece of prior art isdirected to the problem of minimizing standing sound waves in themeasurement chamber.

JP 2002 257 801 discloses an ultrasonic gas analysis sensor which dealswith the problems of avoiding effects on the sound waves due to gas flowrate and diffusion. A measurement chamber with diffusion holes ispositioned in a gas passage tube with a gas inlet and outlet.

EP 1 083 427 discloses a method for determining the gas content of forexample oxygen in breathing gas be means of measuring speed of sound.Problems caused by temperature variations are dealt with bysynchronising sound speed detection with one or a plurality of specifictimes in a respiratory cycle.

EP 1 336 841 discloses a method for determining the temperature in anacoustic gas meter by means of an elongate resistive temperature sensorpositioned in the ultrasound propagation region of the gas meter.

GB 2 195 767 discloses concentration measurement of a substance, such asa liquid, using ultrasonic pulses and detection of an nth echo.

U.S. Pat. No. 5,060,506 discloses a method and an apparatus formeasuring the ratio of gases in a two gas mixture such as a therapeuticoxygen/nitrogen mixture. The gas mixture is passed through a sample tubewithin which ultrasound waves travel in successive bursts of pulses atthe resonant frequency of the transmitter/receiver pair. Between burstsis a quiescent time period having a duration that is long enough toassure dissipation of transients so that standing waves do not form. Thedelay caused by the transit time of the sound through the gas samplegenerates electrical pulses that are translated into an analogue signalwhich is then temperature-corrected. The resulting voltage isproportional to the transit time and thus to the gas composition. Thesample tube is contained inside a larger cylindrical body to enhance thegas flow and provide thermal insulation which is intended to improve theaccuracy of temperature compensation.

U.S. Pat. No. 4,938,066 discloses a method and apparatus dealing withthe problem of length expansion in an acoustic sensor with an ultrasoundtransducer emitting sound pulses that are reflected against a proximatesurface of an Inver rod and against a second surface at the distal endof the invar rod. The time difference between detection of reflectedpulses from the respective surfaces and the known length of the rod areused to calculate the speed of sound.

U.S. Pat. No. 6,481,288 discloses a method and apparatus for measuringthe speed of sound employing a spherical measurement chamber.

SUMMARY OF THE INVENTION

An object of the present invention is to further improve the accuracy insound speed detection based measurement devices and methods in a gasmixture in environments where pressure induced temperature influence isa dominant factor for measurement accuracy.

There are, inter alia, the following aspects of the object and theproblem:

To decrease the influence of pressure variations on the temperature of aportion of a gas mixture currently subjected to measurement.

To provide an appropriate sound pattern from an ultrasound transmitterin the measurement device.

To decrease influence from thermal expansion of material in themeasurement device.

To determine gas concentration in a gas mixture.

To determine flow rate in a gas flow.

In accordance with the invention the above object is achieved byequilibrating temperature change in a portion of the gas or gas mixturethat is currently being subjected to measurement, and thereby suppresstemperature variations in the gas. In other words, the invention solvesthe problem by counteracting and levelling out temperature gradients ina sensor arrangement caused by the measured entity, i.e. the measurementobject in the shape of a gas mixture.

As mentioned in the background, the temperature of the gas changesrapidly during pressure changes, which for example in a typicalapplication of the present invention in medical breathing apparatusesoccur in the tubing system in connection with inspiration or expirationventilation of a patient. Concurrently with the temperature change inthe gas there is a process of levelling out or equilibrating thetemperature change of the gas towards an average temperature by thermalexchange between surrounding matter and the gas.

The invention is based on the insight that the speed of the levellingout process can be increased to such a degree that the influence oftemperature gradients due to pressure changes in the gas are minimizedand even negligible. In accordance with the invention, acoustic gasmeasurement is therefore devised such that thermal exchange is amplifiedand the speed of the levelling out process is matched with predeterminedrequirements on the time lag of equalization of a temperature change inthe gas. This is in contrast with prior art, in which e.g. measurementspeed is increased or sudden temperature changes are compensated for incalculations in order to handle measurement problems due to pressureinduced temperature changes. In the invention, the measurement chamberitself is adapted so as to decrease or even eliminate these measurementproblems.

In accordance with the invention, the thermal exchange is amplified bymaking the gas measured upon come into close contact with solid surfacesof a thermally inert and thermally well conducting matter so that heatcan move freely from the gas to surrounding structure. In animplementation of the inventive concept, the invention is achieved byproviding a measurement chamber that is devised with a structure adaptedto amplify the thermal exchange between a portion of the gas that iscurrently present in the chamber and the chamber defining structure. Thetechnical effect of this is that when fast energy pulses that occur inthe shape of pressure variations are introduced into the system oftubing and connected equipment, the measurement chamber will operate asa low pass filter for the energy pulses and the matter in the chamberwill present small variations around an average temperature. Due to theamplified thermal exchange between the chamber matter and the gas, thegas will also be low pass filtered in this respect and temperaturevariations will be levelled out so fast that they become negligible inthis context.

Preferably, the chamber defining structure is adapted to provide thermalexchange such that a temperature change is equalized to a predeterminedlevel within a predetermined time lag. The time lag is for exampledefined as the time constant for thermal exchange of the chamberdefining structure. The chamber defining structure is for exampleadapted to provide the required thermal exchange efficiency dependent ona predetermined maximum error in gas concentration rate. This isembodied as a method of dimensioning a measurement device within theinventive concept.

The speed of the temperature change levelling out process in factdepends on a number of parameters such as the thermal conductivity ofthe gas, the geometric distance from gas molecules to the surfaces ofsurrounding matter, the thermal inertia and the thermal conductivity ofthe surrounding matter. The geometrical distance and the thermalconductivity have been found to be the dominant parameters with regardto the present invention.

In accordance with a first aspect of the invention, temperatureequilibrium is achieved by designing the measurement chamber with arelation between the shape of a cavity in the chamber and the propertiesof material defining the cavity. According to this aspect, a chamberdefining structure is adapted with a combination of shape, size andmaterial in relation to properties of the gas mixture to be used in aparticular application of the invention. Preferably, this adaptation isdevised such that the time lag for thermal exchange between the gasmixture and the measurement apparatus is less than or equal to (≦) apredetermined maximum allowable time lag for thermal exchange. A maximumallowable time lag for thermal exchange is preferably calculateddependent on a maximum allowable error in the determined proportion of agas in the gas mixture and the magnitude of occurring temperaturevariations in the gas mixture. In an embodiment, the maximum allowabletime lag is calculated also dependent on the time lag of a signal filterfor application on a signal from an ultrasound detector in themeasurement device.

According to a second aspect, the influence from expansion of thematerial in the measurement device is decreased by mounting theultrasound transducer in the measurement chamber such that influencefrom expansion in the transducer is eliminated.

According to a third aspect, structure borne sound originating from theultrasound transducers is reduced by means of an acoustically dampeningsealing member placed between the end surface of the measurement chamberopenings and the active surface of the transducers. The sealing memberis preferably made in silicon rubber.

According to a fourth aspect of the invention, the resolution ofmeasurement in a small size measurement chamber with a short distancebetween ultrasound transducers is improved by allowing an ultrasound tobounce between transducer surfaces and detecting the nth echo. Therebythe ultrasound pulse travels a longer distance.

According to a fifth aspect of the invention, the pattern of ultrasoundtransmitted through the gas in the measurement chamber is devised inorder to decrease transients and render a harmonious transientsteady-state process. This is achieved by repeatedly transmitting burstsof pulse trains and starting each train with a ¼ pulse period. Thisentails that the frequency applied to the transducer is not equal to theresonance frequency of the transducer in a mathematical sense.

Further aspects of the invention are explained in the detaileddescription of embodiments.

In addition to the above mentioned technical effects and advantages, theinvention also entails the following.

A simple temperature sensor with a comparatively large time constant canbe used in the measurement device since the speed of temperature changesis decreased through the temperature equilibrating effect.

The inventive measurement device is an environmentally more friendlytechnical alternative to carbon-combustion cell (fuel cell,electrochemical cell) based sensors. Such sensors have to be replacedonce a year, and produce a significant amount of lead waste.

Furthermore, the invention enables an alternative to paramagneticsensors that are not only expensive but also mechanically sensitive.

In this text the term equilibrating as well as synonymous expressionssuch as equalizing, level out and the like are used to describe theactivity of exchanging thermal energy in order to suppress temperaturegradients in the measured portion of the gas.

Gas and gas mixture are expressions used to describe the gas entity thatis measured upon. A typical gas mixture in medical breathing apparatusapplications is a so called binary gas composed of a normal air mixtureand some other gas such as oxygen or nitrogen.

The invention concerns determining and monitoring the concentration of agas component in a gas mixture. There are various alternativeexpressions for this, e.g. proportion of gases, relative proportions ofgases, gas content and the like.

In methods for determining the proportions of gases in a binary gasmixture, i.e. a mixture of two gases, the following well known relationcan be used. The speed of sound in a gas mixture can be describedaccording to the equation:

$\begin{matrix}{c = \sqrt{\frac{c_{p}^{*}R_{M}T}{c_{v}^{*}M^{*}}}} & \lbrack {{eq}.\mspace{20mu} 1} \rbrack\end{matrix}$where c=speed of sound [m/s]

cp=specific heat at constant pressure [kJ/kgK]

cv=specific heat at constant volume [kJ/kgK]

Rm=gas constant 8314.5 [J/kmolK]

T=temperature [K]

M=molecular weight,

and where c_(p), c_(v) och M (molecular weight) of a gas has beenreplaced with the corresponding values for a mixture of gases taken fromtables of physical properties gases. The replacing quantities c_(p),*c_(v)* och M* depend on the proportion of gases. The currentconcentration of one of the gases can be calculated by resolving theequation for M* in a per se known manner.

The time constant is generally the time required for an instrument toindicate a given percentage of the final reading resulting from an inputsignal; the relaxation time of an instrument. In the case of instrumentssuch as thermometers, whose response to step changes in an appliedsignal is exponential in character, the time constant is equal to thetime required for the instrument to indicate 53.2 percent of the totalchange, that is, when the transient error is reduced to 1/e of theoriginal signal change. Also called lag coefficient. [Source: Webster'son-line dictionary] In the invention a time lag can for example bedefined as the time constant. In the description of embodiments of thepresent invention a time constant tau is defined for the measurementchamber according to this definition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic overview of a measurement system in accordancewith an embodiment of the invention.

FIG. 2 shows an embodiment of the invention with solid structure flangesin the sound propagation region of the measurement chamber.

FIG. 3 shows an embodiment of the invention with a porous structure inthe sound propagation region of the measurement chamber.

FIG. 4 shows an exploded view of a measurement device assembly accordingto an embodiment of the invention.

FIG. 5 shows a cross section of the measurement device assembly in theembodiment of FIG. 4.

FIG. 6 shows a perspective view of a measurement chamber pipe, and

FIG. 7 shows a cross section side view according to the embodiment ofFIG. 5.

FIG. 8 shows an outline of an embodiment with an equilibrating upstreamgas channel portion.

FIG. 9 shows schematically how the transducer is mounted to themeasurement chamber in accordance with an aspect of the invention;

FIGS. 10A-D illustrate how an nth echo of an ultrasound pulse isdetected; and

FIGS. 11A-D illustrate how the emitting transducer is excited inaccordance with an aspect of the invention;

FIG. 12 shows a graph of the temperature interference due to anintroduced pressure transient; and

FIG. 13 shows a graph of the decrease of relative temperatureinterference that is plotted over time normalized with the timeconstant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In preferred embodiments, the invention is realized by means of ameasurement chamber for momentarily housing the gas that is to bemeasured upon. Preferably, the chamber is mounted such that a gas flowpasses through the chamber and measurement samples are madeintermittently on the flowing gas. A functional requirement on themeasurement chamber is that the chamber defining matter shall operate asa thermal buffer and have an efficient thermal exchange with the gasmolecules. The better efficiency in thermal exchange is achieved, themore the speed of the temperature equilibrating process is increased,and the better effect of the invention.

FIG. 1 shows schematically a measurement arrangement 1 in accordancewith the invention. A measurement chamber 2 defined by a chamberdefining structure 5 has a cavity 4 with space for gas in whichultrasound propagate from an ultrasound transmitter to an ultrasoundreceiver. In the embodiment of FIG. 1 the ultrasound propagates betweentwo oppositely mounted first and second ultrasound transducers 6A, 6Bthat have transmitter as well as receiver functionality in a per seknown manner. The chamber is provided with a gas inlet 8 and a gasoutlet 10 devised for passing gas from a gas flow path 12 through thecavity 4 of the measurement chamber 2. A temperature sensor 14 isdevised for detecting the temperature of the gas in the chamber. Thetemperature must be measured in the proper region, i.e. the regionthrough which the sound propagates. This can for example be achieved bymeans of a temperature sensor comprising a thin platinum thread that isstrained is through the measurement region. An alternative is to sensethe temperature at a point having a known relation to the soundmeasurement region and calculating the temperature in the gas.Preferably, a temperature sensor that has as small time constant aspossible should be selected. The exemplifying platinum temperaturesensor has a short time constant and is a suitable choice forrealisation of the invention. However, the invention has the effect thattemperature variations are levelled out by means of efficienttemperature exchange so that the temperature variation is low passfiltered and therefore a simple temperature sensor with a comparativelylarge time constant can be used. A currently preferred embodiment isprovided with a standard NTC type termistor (NTC=Negative TemperatureCoefficient).

In FIG. 1 there is also a schematic block diagram of an electroniccontrol circuit according to one embodiment. A central processing unitCPU 16 is provided with program code specifically adapted to theinvention and is via a temperature signal input 18 and a temperaturesignal amplifier 20 coupled to the temperature sensor 14. The firstultrasound transducer 6A is via an ultrasound receiver signal amplifier22 coupled to an input 24 of a timer stage 26. The second ultrasoundtransducer 6B is via an ultrasound transmitter drive signal amplifier 30coupled to an output 28 of the timer stage 26. The timer stage iscoupled to or is realised by means of the central processing unit 16,and has the function to register and compare the time of arrivingdetector signals from the receiving ultrasound transducer 6A, the timedifference between signals and the time of outputting drive signals forexciting the emitting ultrasound transducer 6B. A digital/analogue (DIA)converter 36 is coupled to the central processing unit for outputtingfor example measurement results on a display, or for producing alarmsignals. The signals from the ultrasound transducers 6A, 6B are alsopassed through or applied in a not shown signal filter that has acertain time lag, usually expressed in terms of a time constant. Thecentral processing unit 16 is further coupled to an I2C-bus 32 forcommunication with other circuit components. A power supply 34 iscoupled to the components as required. The sensor arrangement is thusoperated to send an ultrasound pulse through the gas mixture in thechamber, for example a mixture of air and oxygen, and the propagationtime for the pulse is measured. The measured time corresponds to a timervalue in the processor, for example the number 4534. Together with themeasured temperature of the gas, a gas concentration rate is calculatedin accordance with pre-programmed relations, and an output signal isgenerated in the shape of an analogue voltage for example correspondingto 21-100% oxygen O2.

The cavity 4 of the chamber 2 is defined and delimited by a physicalchamber defining structure 5 designed with an appropriate shape and withan appropriate material and having surfaces that come into contact withgas streaming through the chamber.

The thermal conductivity of the structure determines the speed ofthermal exchange and depends on properties of the material, shape andphysical dimensions of the structure. Thermal conductivity is in the SIsystem of units measured in watts per meter-Kelvin, (W·m⁻¹·K⁻¹), whichfor materials is known from tables. For shape and physical dimensionsthermal conductivity is for example estimated with indirect methods sucha modelling and experimental measuring of dependent parameters. Thematerial of the structure should thus have a high thermal conductivityand preferably be a metal such as aluminium having a thermalconductivity of 236 W·m⁻¹·K⁻¹ or steel with a thermal conductivity ofabout 50 W m⁻¹ K⁻¹. Other materials are also conceivable, for exampleheat conducting plastics added with metal chips. The weight of thethermal buffering material should be selected in relation to the otherdesign parameters and to weight, volume and molecule density of the gascontent in the chamber such that the thermal exchange efficiency issufficient. In accordance with the invention, the available designparameters should be employed so that the chamber defining structure isadapted to provide thermal exchange such that a temperature change inthe gas is equalized to a predetermined level within a predeterminedtime lag. It is in this context useful to define a time lag or a timeconstant for the thermal exchange of the chamber defining structure as ameasure on the thermal exchange efficiency of the chamber.

The inventive concept concerns a method of dimensioning a measurementapparatus in accordance with the invention. The method is indirect inthe sense that it approaches the dimensioning parameters from thedesired accuracy of the measurement result and goes backwards to findrequirements on the dimensioning parameters.

Thus, a maximum allowable error in the determined proportion of a gas inthe gas mixture is determined. The allowable error depends on theapplication and would in an application of the invention in a breathingapparatus for example be in the range of 5% error in the a determinedoxygen concentration (O2-concentration). The magnitude of maximumoccurring pressure variation in the gas mixture is estimated. In theexemplifying application the maximum occurring pressure variation wouldfor example be in the range of 50 cmH2O. In practical ventilation of ahuman patient, the most common pressure variations are in the range of25-30 cmH2O. However, the pressure variations can reach extreme valuesup to about 100 cmH2O for example when the patient coughs.

The magnitude of maximum occurring temperature variation in the gasmixture is estimated by means of a calculation dependent on theestimated magnitude of occurring pressure variation in the gas mixture.For this estimation a model of the heat conduction process is determinedand expressed as an equation comprising a term that represents thecompression work performed by the varying pressure. In an exemplifyingembodiment, this equation is expressed as

$\begin{matrix}{{\rho\; C_{p}\frac{\partial T}{\partial t}} = {{\lambda\;{\nabla^{2}T}} + \overset{.}{p}}} & {{Eq}.\mspace{20mu} 2}\end{matrix}$where T is temperature, p is pressure, ρ is density, Cp is specific heatat constant pressure and λ is heat conductivity.

The following mechanical and thermal parameter values are valid for airat 25° C.:

TABLE 1 Parameter Value ρ 1.18 [kg/m³] C_(p) 1000 [J/kg K] λ 0.024 [W/mK]

In this model example the density ρ is approximated to a constant valuesince comparatively small pressure and temperature variations arestudied. Now solving the equation Eq. 2 for a case without any heattransmission term and an introduced pressure transient Δp=5000 [Pa]=50cmH2O results in the values plotted in the graph of FIG. 12. Thiscorresponds to an infinite size of the measurement chamber or to a heatconductivity λ=0. In FIG. 12 the interference in terms of temperaturevariation is plotted as a function of time where values for time is onthe horizontal axis and values for temperature interference in degreesCelsius is on the vertical axis. As shown in the graph the inputpressure transient of Δp=5000 [Pa]=50 cmH2O entails a very fast decreasein temperature of about 4 degrees Celsius.

Thus in this example, a pressure decrease of 50 cmH2O would cause atemperature variation in the form of a decrease in the range of 4degrees Celsius (° C.). Then it is calculated a maximum allowable timelag for thermal exchange between the gas mixture and the measurementapparatus dependent on the maximum allowable error in the determinedproportion of a gas in the gas mixture and the magnitude of occurringtemperature variations in the gas mixture. In the current example, amaximum allowable time lag, expressed as the time constant, for thermalexchange between the gas mixture and the measurement apparatus would beabout 0.050 seconds. With these requirements the measurement apparatusis adapted such that the time lag for thermal exchange between the gasmixture and the measurement apparatus is less than or equal to (≦) thecalculated maximum allowable time lag for thermal exchange.

Applying the basic result of the dimensioning method, the method furthercomprises the step of selecting a combination of shape, size andmaterial of a measurement chamber in the measurement apparatus inrelation to properties of the gas mixture such that the time lag forthermal exchange between the gas mixture and the measurement apparatusis less than or equal to (≦) the calculated maximum allowable time lagfor thermal exchange.

In embodiments of the invention it is often advantageous to apply asignal filter on signals from the ultrasound detectors, and to also takeaccount of the time lag, preferably the time constant, of the filter indimensioning the measurement apparatus. An embodiment of the methodtherefore further comprises the step of selecting a time lag of a signalfilter for application on a signal from an ultrasound detector in themeasurement apparatus dependent on predetermined requirements on theresponse time of the measurement apparatus. Consequently this embodimentcomprises the maximum allowable time lag for thermal exchange betweenthe gas mixture and the measurement apparatus to be calculated alsodependent on the time lag of a signal filter for application on a signalfrom an ultrasound detector in the measurement apparatus.

In an embodiment in which account is taken of also the time lag of thesignal filter, calculating a maximum allowable time lag for thermalexchange between the gas mixture and the measurement apparatus wouldcomprise the following steps of:

calculating the magnitude of occurring error in the determinedproportion of a gas in the gas mixture dependent on the magnitude ofoccurring temperature variation in the gas mixture;

calculating a maximum allowable detected temperature variation due topressure variation;

determining a relation between the maximum allowable detectedtemperature variation and the maximum occurring temperature variation inthe gas mixture due to pressure variations;

determining a relation between the time lag for thermal exchange betweenthe gas mixture and the measurement apparatus and the time lag of asignal filter for application on a signal from an ultrasound detector inthe measurement apparatus;

determining a relation between said temperature relation and said timelag relation; and

calculating a maximum allowable time lag for thermal exchange betweenthe gas mixture and the measurement apparatus dependent on said relationbetween said temperature relation and said time lag relation.

As mentioned above, a preferred measure on the time lag is the timeconstant, i.e. according to the definition the time required to reach63.2 percent of the total thermal exchange.

Having determined the requirements on the thermal exchange properties ofthe measurement apparatus, the inventive concept comprises differentapproaches for achieving the sufficient thermal exchange efficiency.

In one embodiment the required thermal exchange speed is achieved bydecreasing or minimizing, in relation to other design parameters, thedistance between gas molecules and surfaces of the matter that definesthe cavity. More specifically, that is the matter that surrounds or issurrounded by the gas in the chamber cavity. In one variety of thisembodiment, the surface area of the thermal buffering material isenlarged in the sound propagation region of the chamber cavity.

For example, as illustrated in the embodiment of FIG. 2, the measurementchamber 2 can be provided with flanges 38 of thermal buffering materialpositioned in the sound propagation region. Preferably the flanges 38are mounted in parallel with the direction of sound propagation 40between transducers 6A, 6B in order not to decrease the sound pressure.Similarly, the main gas flow direction 42 through the measurementchamber should be in parallel with the flanges in order not to disturbthe flow rate through the chamber. In FIG. 2 the gas flow direction 42is into the paper, and thus orthogonal to the sound propagationdirection 40. Experimental tests with a measurement chamber providedwith steel flanges having a thickness of about millimeter (mm) and anintermittent internal distance of about 2 millimeters (mm) have shownthat temperature variations in ventilator operation were evened out tosuch an extent that temperature related effects were negligible. Nomeasurement and compensation of the temperature variations were requiredand only a relatively slow measurement of the basic gas temperature wasneeded in order to achieve accurate results in the calculation of gasconcentration.

In another exemplifying embodiment shown in FIG. 3, the chamber 2 is inits sound propagation region provided with a porous structure 44 ofsolid material such as wire wool or a sponge structure, for example madeof steel or some other metal or similarly heat conductive material. Thegas flows through and surrounds the porous structure 44 and thus a smalldistance from gas molecules to structure surfaces is achieved. Theporous structure should be provided such that ultrasound travels throughthe porous structure without any significant disturbance and with asmall flow resistance.

In a third exemplifying embodiment the chamber cavity is shaped as apipe with a small diameter, which renders a small distance from gasmolecules in the cavity to the inner walls of the pipe. Preferably, theinner pipe shape is cylindrical but other cross sections are alsoconceivable. In a practical implementation of this embodiment a partialgas flow is diverted from the main gas flow to the measurement chamberin order to thermally buffering and measuring an appropriate gas volume.This concept is applied in a presently preferred embodiment of theinvention shown in FIGS. 4 to 7. A model has been determined in order tofind a suitable diameter of the pipe shaped chamber cavity. This modelis based on studying a relative level of interference on the measurementaccuracy due to temperature variations caused by pressure transients. Itis assumed that the relative temperature interference level T(r,t)decreases exponentially according to the equation:

$\begin{matrix}{{T( {r,t} )} = {\Delta\; T_{0}{\exp( {- \frac{t}{\tau}} )}{g( \frac{r}{r_{0}} )}}} & {{Eq}.\mspace{20mu} 3}\end{matrix}$

-   where r0 is the inner radius of a cylinder, r is a radius in the    space in the cylinder, i.e. in the pipe shaped chamber, g(r/r₀) is a    temperature distribution in the space in the cylinder and τ is a    time constant.

The time constant τ is

$\begin{matrix}{\tau = {0.1729\;\frac{\rho\; C_{p}}{\lambda}r_{0}^{2}}} & {{Eq}.\mspace{20mu} 4}\end{matrix}$where ρ is density, Cp is specific heat at constant pressure and λ isheat conductivity with values as described in Table 1 above.

The graph in FIG. 13 shows an example of a relation in the shape of thecurve for the decrease of relative temperature interference that isplotted over time normalized with the time constant. After 1 timeconstant the interference level has decreased to about 36 percent of theinitial level.

Using these equations in modelling calculations show relations betweendifferent pipe radius r0 [millimeters] and the relative temperatureinterference level after a time constant of τ (tau) [s] due to pressurevariations compared with a nominal level without any temperatureequilibrating according to the invention. In the below table, theseparameters are listed at two points in time t=0.01 [s] and I=0.1 [s],respectively, after an introduced pressure transient. For example, it isshown in the table that a pipe with a radius of 3.0 mm has a timeconstant tau=0.0765 and a relative temperature interference level of0.877 after 0.01 seconds and 0.271 after 0.1 seconds.

TABLE 2 r₀ [mm] tau [s] exp(−0.01/tau) exp(−0.1/tau) 1.0 0.0085 0.3080.0000078 1.5 0.0191 0.593 0.00536 2.0 0.0340 0.745 0.0528 2.5 0.05310.828 0.152 3.0 0.0765 0.877 0.271 4.0 0.136 0.929 0.479 5.0 0.213 0.9540.625 6.0 0.306 0.968 0.721 7.0 0.417 0.976 0.787 8.0 0.544 0.982 0.832

It can be shown that for a gas mixture of 50 percent O2 and 50 percentN2 the error in determined O2 concentration is about 2.5 percentageunits per degree Celsius of temperature variation. So, in the previousexample with a radius of 3.0 mm and a relative interference level of0.877 after 0.01 seconds, the error in the determined O2 concentrationwould be ΔXO2=2.5×0.877≈2.2 percent.

In an application of the invention in an O2-sensor for a breathingapparatus, pressure variations with extreme values of up to about 100cmH2O have to be accounted for in the dimensioning process. This would,in the same manner as in the calculations above result in temperaturevariations of up to about 8 degrees Celsius, which in its turn bycalculating 2.5 [percentage units error in O2 concentration/degreeCelcius temperature variation]×8 [degrees Celcius] results in an errorinterference of up to about 20 percentage units of O2 due to pressureinduced temperature variation. By means of signal filtering thisinterference can be reduced to a more tolerable level of about 3percents. That is the filter shall reduce the error to a fraction with areduction factor of 3 percent/20 percent=0.15. A signal filter is thusapplied on the signal from the ultrasound sensor. The signal filterlimits the speed of the ultrasound sensor with regard to signals thatoriginate from real variations in O2 concentration. With a large filtertime constant τf the smaller is the influence on the concentration frompressure induced temperature variation. With this configuration, theerror in the measured O2 concentration is thus affected on one hand bythe thermal time constant τ of the measurement chamber and on the otherhand by the signal filter time constant τf. Using a more detailedversion of the dimensioning method with the signal filter brieflydescribed above and applied for determining required dimensions of ameasurement chamber, it is determined a relation between the timeconstant τf of the signal filter, the time constant τ of the measurementchamber and a reduction factor η that corresponds to a reduction factorof how much the filter reduces the maximum interference. From thisrelation (not shown in detail) it is in this example determined that fora reduction factor 0.15 the time constant τ of measurementchamber≦0.235×time constant of the signal filter τf. In practice, theconnection τ≦0.235×τf is for example found as a point on the graph thatdescribes the relation of the interference reduction factor as afunction of the time constant ratio, i.e. η(τf/τ). With a slow filterhaving a time constant of 20 seconds, the maximum time constant of themeasurement chamber would thus be 0.235×20=4.7 seconds. Calculating theinner diameter of a metal pipe as a decisive dimensioning parameter andassuming the gas mixture to be air, the resulting requirement is thatthe maximum inner diameter is 47 millimeters.

When carrying out the invention there are different design factors andparameters to consider in order to provide a thermal exchange effectwith a sufficient efficiency in accordance with the invention. In oneembodiment of the inventive concept a selected structural design isdimensioned with regard to for example diameter, wall thickness, weightor heat conductivity by means of the following method. The variation ofpressure that can occur in a specific application of the invention ismeasured, with particular attention given to unfavourable pressurecases. A simulation of consequences of one or more selected designparameters on for example time constant and temperature interferencelevel is carried out and a maximum temperature interference level isdetermined. The temperature deviation or temperature interference levelis translated into an error in a resulting calculated gas concentrationor gas flow. A maximum allowable error, thus alternatively expressed asdeviation or interference level, for a resulting measured gasconcentration level is predetermined, for example maximum 3% error inO2-rate. Thereafter, the selected design parameter, for example diameterof a pipe, is calculated dependent on the predetermined maximum gasconcentration error. If for example the required heat conductivity isthe selected design parameter to be dimensioned, the correspondingcalculation is made also dependent on a for example a predetermined pipediameter. This concept is similar to the modelling example given above.

FIG. 4 shows an exploded view of main parts of an embodiment of theinvention realised as a measurement device assembly comprising a sensorhousing 46 with a measurement chamber that is not visible in FIG. 4, acircuit board 48 with connectors to external wiring and a protectivecover 50 devised to cover the sensor housing 46 and the circuit board48. The sensor housing 46 is devised to be mounted on a main gas flowpipe 52 covering and partly intruding into the main gas flow via anopening 54 in the gas flow pipe 52. A part of the gas flow is therebyconducted into the measurement chamber of sensor housing 46.

FIG. 5 shows a cross section of the measurement device assembly mountedon a main gas flow pipe 52 according to FIG. 4. A sensor housing 46 isprovided with a lip member 56 projecting into the main gas flow pipe 52and conducting gas to a measurement chamber 2. The measurement chamber 2is spatially defined by a cylindrical pipe 58 comprising mutuallyopposing elongate inlet and outlet openings 60 for the gas positionedalong the mantle of the cylinder. The elongate form of the inlet andoutlet opening are devised to minimize affecting the ultrasoundpropagation pattern that may occur in connection with sudden areachanges in a gas flow. More specifically, sound pulse bounces at thearea changes are decreased or even avoided, and thereby destructiveinterference with the sound propagation patter due to such sound pulsebounces is avoided. In operation, the gas flows along the lip member andto and from the pipe cavity via the inlet and outlet openings 60. InFIG. 5 only one of the inlet and outlet openings is visible. Ultrasoundtransducers 6A, 6B are positioned at each end opening of the pipe suchthat the active surface of the transducer covers the end opening of thepipe at each respective end. A washer 62, having the function of adampening seal, seals to prevent leakage between the active surface ofthe transducer and the chamber pipe 58. O-rings 64 seal between thechamber pipe 58 and the sensor housing 46 and a collar shaped sealing 66seals between sensor housing 46 and main gas flow pipe 52. A springyshackle piece 68 at each side straps the transducers 6A, 6B against thechamber pipe 58. It is clear from FIG. 5 that most of the components inthe sensor housing appear in pairs that are mounted in a symmetricalfashion around the chamber pipe 58. A circuit board 48 is placed on topof the sensor housing 46 and finally the assembly is covered by aprotective cover 50.

FIGS. 6 and 7 show the chamber pipe 58 in more detail. The chamber pipeis at one of the ends provided with a flange 82 having the function of aposition stop when the pipe 58 is mounted in the sensor housing. At eachend of the pipe 58 there is a recess 84 for receiving the O-ring seal 64(Cf. FIG. 5). Elongate inlet and outlet openings 60 are as mentionedabove provided along the mantle of the pipe 58. The dimensions of thechamber pipe are preferably optimized with regard to temperaturevariations as well as to signal/noise level. This exemplifyingembodiment is designed for an inspiration channel of a breathingapparatus with a typical gas flow rate in the range of 0-200 l/min inthe main flow through the inspiration channel, a typical operating gastemperature in the range of 15-50° C. and typical pressure variations inthe range of 0-140 cmH2O. This embodiment of the chamber pipe preferablyhas an inner diameter in the range of 9 mm and an outer diameter in therange of 12 mm, rendering a material thickness of about 3 mm in themajor part of the chamber wall and a maximum distance from a gasmolecule to an inner surface of about 4.5 mm. The material is stainlesssteel having heat conductivity in the range of 50 W m⁻¹ K⁻¹ln anembodiment of the invention applied in an environment where the gas flowis large in relation to the volume of the measurement chamber cavity,there is preferably provided an upstream gas channel portion devised forequilibrating temperature gradients in accordance with the inventiveconcept. Thereby the risk that the gas portion in the chamber isexchanged with a speed that exceeds the speed of the temperatureequilibration in the chamber is eliminated. Such an upstream gas channelportion is thus preferably devised with a heat conductive material in across section dimension that is small compared with the main flowdimensions. FIG. 8 shows an outline drawing of this embodiment, with atemperature equilibrating channel portion 90 positioned upstream themeasurement chamber cavity 4 and devised to level out the temperature T0of inflowing gas before it enters the measurement chamber. In theembodiment of previously explained FIG. 5, the lip member 56 togetherwith the surrounding structure forms such a temperature equilibratingchannel portion.

Another aspect in acoustic sensors in accordance with the inventiondeals with the problem of expansion in the measurement chamber.Measuring the speed of sound c by means of an acoustic sensor is basedon measurement of the time t it takes for a sound pulse to travel aknown distance s, and calculating c=s/t. It is therefore required tohave an accurate and stable measure of the measurement distance. Forexample, in measuring gas concentration via sound speed detection in agas mixture of air and oxygen on a measurement distance of 18 mm achange of 10 micrometers in the measurement distance renders an error ofabout 1% in the oxygen concentration unit. Expansion of the measurementdistance can occur due to thermal expansion in the chamber or in thetransducer and due to pressure changes in the measurement chamber.

The thermal expansion occurs in the chamber enclosing structure and inthe ultrasound transducer itself. The material for the chamber enclosingstructure is in accordance with the invention dealt with by selecting amaterial with as low linear temperature expansion coefficient aspossible in relation to other design parameters. For example, aluminiumwith the linear thermal expansion coefficient of 22.2 (m/m.K×10⁻⁶) orsteel with 13.0 (m/m.K×10⁻⁶) can be used for the chamber.

The ultrasound transducer is composed of layers of different materialswith an active ultrasound emitting surface on an adaptation material, alayer of dampening material and a layer of silicone rubber all mountedon a flange that is normally used for mounting the transducer againstthe rim of a recess. All these layers expand or shrink dependent ontemperature changes in the environment or in the gas, or due toultrasound generation and thereby incur errors in measurements. Inaccordance with the invention, and as shown in the schematic drawing ofFIG. 9, this effect is eliminated by mounting the transducers such thatthe active surfaces 94 of the respective transducer 6A/6B cover the endopenings of the pipe shaped chamber cavity 4. Dimension changes in thematerial making up the transducer are thereby negligible since theposition of the active surface is kept constant. The active surface 94rests against a thin acoustically dampening seal in the shape of awasher 62 positioned between the end cross section of the chamber wall97. The dampening seal washer 62 is preferably made of silicone rubber,for example with a thickness of about 0.7 millimeters. The dampeningseal washer 62 addresses a fourth aspect of the invention, namely toeliminate structure borne sound. Due to its small thickness the siliconeseal has a negligible length expansion due to pressure as well astemperature.

Further, the transducer is mounted with a spring device 102 pressing thetransducer against the dampening seal washer 62. In FIG. 9, the springis illustratively seated against a shackle or strap 104 that is rigidlyfastened in relation to the chamber structure. In the embodiment shownin FIG. 6, the shackle piece 68 is itself embodying a spring. The springdevice should be devised with a spring force that exceeds the largestpressure pulses that occur in the measurement chamber and therebyeliminates displacement of the transducer and pressure dependentexpansion of the measurement distance.

An advantage with this embodiment is that it enables a small sizemeasurement chamber. The small size simplifies the temperaturemeasurement in connection with gas concentration measurement, since asmall sound propagation region renders a more homogenous gastemperature. This in its turn enables that the gas temperature can bemeasured in a point instead of over the whole region. This concept canbe used as illustrated in an embodiment of the present invention orindependently in conjunction with other configurations of ultrasoundtransducers.

Another approach of dealing with expansion in the chamber material is todesign the measurement chamber with a spherical shape, entailing thatthe thermal expansion of the chamber enclosure will be equal in alldirections. A drawback with this embodiment is that a sphere is thegeometrical shape that renders the smallest possible surface area inrelation to the volume. This embodiment is therefore preferably providedwith extra thermally buffering material in order to achieve asufficiently efficient thermal exchange with the gas content of thechamber. This may for example be realised by means of thicker chamberwalls, flanges or a porous structure inside the chamber cavity.

A further aspect of the invention is concerned with the problem ofachieving a satisfying resolution in the ultrasound measuring in spiteof a short measurement distance. This is solved in accordance with theinvention by allowing a sound pulse to bounce between the pair oftransducers before a read out of the travel time is made. In effect,thus a reflected signal that has traveled a longer distance is detectedand timed. FIGS. 10A-10D illustrate the invention. FIG. 10A illustratesfor the sake of the example an emitting ultrasound transducer 106 thatsends an ultrasound pulse over a measurement distance 110 of for example6 centimeters to a receiving transducer 108. FIG. 10B illustrates thesound pulse emitted at 1 with a first emitted amplitude and received at2 with a second lower amplitude due to energy loss during the travelthrough the gas. In FIG. 100 the emitting and the receiving transducersare positioned with a shorter intermediate distance of for example 2centimeters. The sound pulse is as illustrated by the arrows allowed tobounce or echo twice between the transducers and is detected by thereceiving transducer after having traveled three times the distancebetween the transducers. As illustrated in FIG. 10D, the receiver ispreferably dampened during emission of the sound pulse in time period 1in order to counteract interfering oscillations. The pulse bounces firsttime in period 2 and second time in period 3 whereupon it is detected bythe receiving transducer. In general, the nth echo is detected andappropriate consideration is taken to signal dampening. In a preferredembodiment, however, dependent on the design of the measurement chamber,the second echo is detected. Detection of an nth echo, e.g. the second,of the ultrasound pulse entails that the ultrasound travels a longerdistance and thus improves the resolution of measurements in a smallsize measurement chamber.

Yet another aspect of the invention deals with the problem of achievingappropriate signal patterns from the sensor arrangement. In accordancewith an embodiment of the invention, the emitting transducer is operatedto emit an ultrasound pulse with a repetition frequency of 200 Hz. Thetransducer is in this connection excited to oscillate and emit pulsesthat in fact are bursts of pulse trains for example comprising 3 pulseseach having the duration of 2.25 periods thus corresponding to 0.0050seconds. In order to decrease transients and render a harmonioustransient steady-state process, the transducer exciting voltage iscontrolled to oscillate and have a high voltage only during theelevation period of the oscillations of the transducer. That is when thetransducer output has a positive derivative and thus the firstoscillation of the exciting voltage has a length of only ¼ pulse period.This is illustrated in FIGS. 11A-11D. FIG. 11A is a screen dump from anexemplifying oscilloscope measurement illustrating how a sound pulsebounces between the transducer surfaces in the measurement chamber asdetected by the receiving transducer. The sound has been dampened outafter about 2.4 milliseconds and a new pulse emission can start. In FIG.11B the upper plot shows detected ultrasound pulses and lower plot showsthe excitation voltage input to the emitting transducer. The time fromthe start of the emission until for example the second echo is receivedby the receiver is measured by means of a timer in the microprocessor asexplained above. FIG. 11C shows a partial enlargement of the excitationoscillation in FIG. 11B. The first oscillation is only a ¼ pulse periodlong whereas the subsequent oscillations have a length of a ½ pulseperiod, thus the applied signal deviates from the resonance frequency ofthe transducer in a mathematical sense. This is also shown in theschematic illustration of the excitation oscillation in relation to theoutput of the emitting transducer. The exciting voltage is in accordancewith FIG. 11D input to the transducer in the shape of a burst of squarewave oscillations 112 starting with a ¼ pulse period thus having a highvoltage during positive derivative portions of the transducer outputsignal 114. This concept can be used as illustrated in an embodiment ofthe present invention or independently in conjunction with otherconfigurations of ultrasound transducers.

The inventive concept can also be applied in measurement of gas flow bymeans of ultrasound. An ultrasound based flow meter makes use of thedifference between the speed of sound upstream and downstream in ameasurement region. The average of an upstream measurement value and adownstream measurement value gives an indirect measure on the gascomposition and the gas temperature. With a temperature buffer accordingto the invention applied in the measurement region of a flow meter, themeasurement values will be a measure only on flow and gas composition.This can for example be used in determining functional residual capacity(FRC) of the lungs of a patient. The time of specific measurementsamples does not have to be synchronised with temperature measurements.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventor to embody within the patentwarranted heron all changes and modifications as reasonably and properlycome within the scope of his contribution to the art.

1. A method of dimensioning a measurement apparatus for determining theproportion of gases in a gas mixture by means of ultrasound detection,comprising the steps of: determining a maximum allowable error in thedetermined proportion of a gas in the gas mixture; estimating themagnitude of maximum occurring pressure variation in the gas mixture;calculating the magnitude of maximum occurring temperature variation inthe gas mixture dependent on the estimated magnitude of occurringpressure variation in the gas mixture; calculating a maximum allowabletime lag for thermal exchange between the gas mixture and themeasurement apparatus dependent on the maximum allowable error in thedetermined proportion of a gas in the gas mixture and the magnitude ofoccurring temperature variations in the gas mixture; adapting themeasurement apparatus such that the time lag for thermal exchangebetween the gas mixture and the measurement apparatus is less than orequal to (≦) the calculated maximum allowable time lag for thermalexchange.
 2. The method of claim 1, further comprising selecting acombination of shape, size and material of a measurement chamber in themeasurement apparatus in relation to properties of the gas mixture suchthat the time lag for thermal exchange between the gas mixture and themeasurement apparatus is less than or equal to the calculated maximumallowable time lag for thermal exchange.
 3. The method of claim 1,further comprising selecting a time lag of a signal filter forapplication on a signal from an ultrasound detector in the measurementapparatus dependent on predetermined requirements on the response timeof the measurement apparatus.
 4. The method of claim 3, wherein themaximum allowable time lag for thermal exchange between the gas mixtureand the measurement apparatus is calculated also dependent on the timelag of a signal filter for application on a signal from an ultrasounddetector in the measurement apparatus.
 5. The method of claim 1, whereincalculating a maximum allowable time lag for thermal exchange betweenthe gas mixture and the measurement apparatus comprises the steps of:calculating the magnitude of occurring error in the determinedproportion of a gas in the gas mixture dependent on the magnitude ofoccurring temperature variation in the gas mixture; calculating amaximum allowable detected temperature variation due to pressurevariation; determining a relation between the maximum allowable detectedtemperature variation and the maximum occurring temperature variation inthe gas mixture due to pressure variations; determining a relationbetween the time lag for thermal exchange between the gas mixture andthe measurement apparatus and the time lag of a signal filter forapplication on a signal from an ultrasound detector in the measurementapparatus; determining a relation between said temperature relation andsaid time lag relation; calculating a maximum allowable time lag forthermal exchange between the gas mixture and the measurement apparatusdependent on said relation between said temperature relation and saidtime lag relation.
 6. The method of claim 1, wherein the time lag is thetime constant.